1. Field of the Invention
The present invention generally relates to an impedance matching circuit for facilitating impedance matching between the characteristic impedance of a cable and the input impedance at the input terminal of a receiver for data transmission and, more particularly, to an impedance matching circuit with adjustable resistance for facilitating impedance matching between the characteristic impedance of the cable and the input impedance at the input terminal of a receiver for data transmission even when the characteristic impedance of the cable varies.
2. Description of the Prior Art
FIG. 1 is a schematic diagram showing a data transmission system. In FIG. 1, the data transmission system comprises two portions: a transceiver TX 10 and a receiver RX 12, where a cable 14 is interposed between the transceiver TX 10 and the receiver RX 12 for communication. In general, a cable has a characteristic impedance ZΦ. If the input impedance Zin, at the input terminal of the receiver RX 12 does not match the characteristic impedance Z101 of the cable 14, signal reflection may occur which may distort signals. Therefore, the input impedance Zin of the receiver RX 12 must be properly adjusted to match the characteristic impedance Z101 of the cable 14, so as to reduce signal reflection and prevent signals from distortion.
FIG. 2A to FIG. 2D are schematic diagrams showing various conventional impedance matching circuits in accordance with the prior art. In FIG. 2A, Z101 denotes the characteristic impedance of a cable 202, Zin denotes the input impedance 206 viewed at the input terminal of the receiver RX 208, and R101 denotes a stable resistor 204 interposed between the input terminal of the receiver RX 208 and a voltage source Vdd. Generally, the input impedance Zin 206 at the input terminal of the receiver RX 208 is relatively large. More particularly, the resistance of the input impedance Zin 206 is much larger than that of the stable resistor R101 204. Hence, the parallel connection of the stable resistor R101 204 and the input impedance Zin 206 results in a resistance value approximately equal to that of the stable resistor R101 204. When the resistance of the stable resistor R101 204 is determined to be equal to that of the characteristic impedance Z101 of the cable 202, impedance matching can be achieved.
In FIG. 2B, Z101 denotes the characteristic impedance of the cable 212, Zin denotes the input impedance 216 viewed at the input terminal of the receiver RX 218, and R101 denotes the stable resistor 214 interposed between the input terminal of the receiver RX 218 and the ground. Generally, the input impedance Zin 216 at the input terminal of the receiver RX 218 is relatively large. More particularly, the resistance of the input impedance Zin 216 is much larger than that of the stable resistor R101 214. Hence, the parallel connection of the stable resistor R101 214 and the input impedance Zin 216 results in a resistance value approximately equal to that of the stable resistor R101 214. When the resistance of the stable resistor R101 204 is determined to be equal to that of the characteristic impedance Z101 of the cable 212, impedance matching can be achieved.
In FIG. 2C, Z101 denotes the characteristic impedance of a cable 222, and Zin denotes the input impedance 226 viewed at the input terminal of the receiver RX 228. The input terminal of the receiver RX 228 is connected to the drain of a p-channel MOSFET (abbreviated as “PMOS” hereinafter) 224. The source of the PMOS 224 is connected to a voltage source Vdd, while the gate of the PMOS 224 is connected to the control terminal of a feedback control circuit 225. A precise resistor Rext 227 is interposed between the signal terminal of the feedback control circuit 225 and the voltage source Vdd. Reff denotes the equivalent resistance viewed at the drain of the PMOS 224, therefore the resistance of the precise resistor Rext 227 is expressed as Rext=α·Reff, where the value of α is controlled by the feedback control circuit 225. Generally, the input impedance Zin 226 at the input terminal of the receiver RX 228 is relatively large. More particularly, the resistance of the input impedance Zin 226 is much larger than the equivalent resistance Reff viewed at the drain of the PMOS 224. Hence, the parallel connection of the equivalent resistance Reff and the input impedance Zin 226 results in a resistance value approximately equal to the equivalent resistance Reff When the equivalent resistance Reff is determined to be equal to that of the characteristic impedance Z101 of the cable 222, impedance matching can be achieved.
In FIG. 2D, Z101 denotes the characteristic impedance of a cable 232, and Zin denotes the input impedance 236 viewed at the input terminal of the receiver RX 238. The input terminal of the receiver RX 238 is connected to the drain of an n-channel MOSFET (abbreviated as “NMOS” hereinafter) 234. The source of the NMOS 234 is connected to the ground, while the gate of the NMOS 234 is connected to the control terminal of a feedback control circuit 235. A precise resistor Rext 237 is interposed between the signal terminal of the feedback control circuit 235 and the ground. Reff denotes the equivalent resistance viewed at the drain of the NMOS 234, therefore the resistance of the precise resistor Rext 237 is expressed as Rext=β·Reff where the value of β is controlled by the feedback control circuit 235. Generally, the input impedance Zin 236 at the input terminal of the receiver RX 238 is relatively large. More particularly, the resistance of the input impedance Zin 236 is much larger than the equivalent resistance Reff viewed at the drain of the NMOS 234. Hence, the parallel connection of the equivalent resistance Reff and the input impedance Zin 236 results in a resistance value approximately equal to the equivalent resistance Reff. When the equivalent resistance Reff is determined to be equal to that of the characteristic impedance Z101 of the cable 232, impedance matching can be achieved.
From FIG. 2A to FIG. 2D, the stable resistor R101 and the precise resistor Rext have to change as the characteristic impedance Z101 of the cable varies. When there are a considerable number of cables, the number of the stable resistors increases as the number of cables increases, resulting in increased fabrication cost and complexity of the impedance matching circuit.
FIG. 3 is a schematic diagram showing another conventional impedance matching circuit in the prior art. In FIG. 3, Rcur denotes a built-in/external bias resistor 302 for providing the transistor mib 304 with the current Ibias. A current mirror circuit is composed of the transistor mdrz 306, the transistor mb7 308, the transistor mdlz 310, the transistor mdri 312, the transistor ma7 314, the transistor mdli 316 and the transistor mib 304. Since all the gates of the above transistors are connected together, the current in the current mirror is proportional to the bias current Ibias according to the W/L ratio of the transistors.
The gate voltage Vref of both the transistor muri 318 and the transistor mulz 320 is a reference voltage, the potential level of which is ΔV lower than that of the voltage source Vdd. The transistor muli 322, the transistor muri 318, the transistor mulz 320 and the transistor murz 324 are used for level-shifting, that is, making the gate voltage Vref of the transistors decrease to a voltage value approximately equal to the threshold voltage and then outputting an output voltage (i.e., as a source follower).
An operational amplifier with an output voltage Voa is composed of the transistor mal 326, the transistor ma2 328, the transistor ma3 330, the transistor ma4 332, and the transistor ma5 334. The gate voltage Vref is level-shifted by the transistor muri 318 and then applied to the gate of the transistor ma2 328 through the node ka2. For the output voltage Voa, a negative feedback circuit (where the capacitor mca 340 serves as a frequency compensation capacitor for stabilizing the operational amplifier) is formed of the transistor mna2 336, the transistor mna1 338, the gate voltage Vref, and the node ka1. Hence, the voltage at the node ka1 is equal to that at the node ka2, where the former is a voltage obtained by level shifting the voltage Vext and the latter is a voltage obtained by level shifting the voltage Vref Therefore, the voltage Vext is equal to voltage Vref.
Another operational amplifier with an output voltage Vob is composed of the transistor mb1 342, the transistor mb2 344, the transistor mb3 346, the transistor mb4 348, and the transistor mb5 350. The gate voltage Vref is level-shifted by the transistor mulz 320 and then applied to the gate of the transistor mb2 344 through the node kb2. For the output voltage Vob, a negative feedback circuit (where the capacitor mcb 354 serves as a frequency compensation capacitor for stabilizing the operational amplifier) is formed of the transistor mz0 352, the voltage Vxx, the transistor murz 324, and the node kb1. Hence, the voltage at the node kb1 is equal to that at the node kb2, where the former is a voltage obtained by level shifting the voltage Vxx and the latter is a voltage obtained by level shifting the voltage Vref. Therefore, the voltage Vxx is equal to voltage Vref.
The gate of the transistor mna2 336 is connected to the gate of the transistor mnb2 356. Therefore, the current flowing through the transistor mna2 336 is equal to the current flowing through the transistor mnb2 356, and the current flowing through the resistor Rext 358 is equal to the current flowing through the transistor mz0 352, which means that the resistance value of the resistor Rext 358 is equal to the equivalent resistance of the transistor mz0 352.
The circuit as shown in FIG. 3 is characterized in that Vext=Vref=Vxx and that the current flowing through the resistor Rext 358 is equal to the current flowing through the transistor mz0 352. Therefore, the equivalent resistance of the transistor mz0 352 can be regarded equal to the resistance value of the resistor Rext 358, even though it takes two operational amplifiers to meet the above conditions.
Let us assume that the width of the transistor mz0 352 is equal to Wp, the width of the transistor mlp1 360 is equal to 10Wp, the width of the transistor mlp2 362 is equal to Wp, the width of the transistor mnb2 356 is equal to Ws, the width of the transistor mnx 364 is equal to 11Ws and the gate of the transistor mnb2 356 is connected to the gate of the transistor mnx 364. As a result, the current flowing through the transistor mnx 364 is 11 times the current flowing through the transistor mnb2 356, and the current flowing through the transistor mlp1 360 is 10 times the current flowing through the transistor mz0 352. In addition, the current flowing through the transistor mlp2 362 is equal to the current flowing through the transistor mz0 352 (because the gate of the transistor mlp1 360, the gate of the transistor mlp2 362 and the gate of the transistor mz0 352 are connected). Therefore, the equivalent resistance viewed at the node datab towards the voltage source Vdd is one tenth of the equivalent resistance of the transistor mz0 352 and the equivalent resistance viewed towards the ground approaches infinity. Accordingly, the equivalent resistance at the node datab is equal to ( 1/10)*Rext//infinity=( 1/10)*Rext. (wherein the term “//” means parallel)
However, there are still some problems related to the prior art impedance matching circuit in that: (1) the resistance for impedance matching of the impedance matching circuit as well as the resistor Rext should change when the characteristic impedance of the cable varies; (2) two operational amplifiers are required to complete a negative feedback circuit so that the fabrication cost as well as the complexity may increase; and (3) the resistance value for impedance matching of the impedance matching circuit can not be changed by simply changing the voltage Vref of the impedance matching circuit.